MAJOR CUSTOMERS, PARTNERSHIPS AND STRATEGIC ALLIANCES - Sanofi S.A. (Details)	1 Months Ended	12 Months Ended			29 Months Ended
	Jan. 31, 2014 USD ($) program	Dec. 31, 2021 USD ($) product milestone $ / shares	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Dec. 31, 2021 USD ($)	Aug. 31, 2019 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Accounts receivable		$ 6,013,000	$ 5,224,000		$ 6,013,000
Revenues		110,701,000	118,192,000	$ 102,428,000
Net loss		$ (178,297,000)	$ (121,122,000)	$ (95,419,000)
Earnings per share, basic (in dollars per share) | $ / shares		$ (1.23)	$ (0.90)	$ (0.85)
Sanofi S.A.
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of research programs | program	2
Revenues under agreement	$ 20,000,000
Number of products approved | product		0
Revenues		$ 3,114,000	$ 5,322,000	$ 22,680,000
Collaborative arrangement, increase (decrease) in revenue			2,200,000
Collaborative arrangement, increase (decrease) in net income (loss)			$ 2,200,000
Collaborative arrangement, net income (loss) per share | $ / shares			$ 0.02
Sanofi S.A. | Collaboration and license agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement		$ 20,000,000
Number of milestones included in transaction price | milestone		0
Collaborative arrangement transaction price		$ 96,300,000
Collaborative arrangement estimated reimbursable service costs	62,800,000
Accounts receivable		600,000	$ 1,000,000		600,000
Deferred revenue		1,100,000	1,200,000		1,100,000
Recognition of milestone		100,000
Sanofi S.A. | Collaboration and license agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received	276,300,000
Sanofi S.A. | Collaboration and license agreement | Achievement of specified clinical development and regulatory milestones | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received	115,800,000
Sanofi S.A. | Collaboration and license agreement | Achievement of specified sales milestones | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Development and sales-based milestone payments to be received	$ 160,500,000
Sanofi S.A. | Collaboration and license agreement | Milestone One
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone payments received		6,000,000
Milestone revenue receivable						$ 6,000,000
Recognition of milestone					5,900,000
Sanofi S.A. | Collaboration and license agreement | Milestone Two
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable		7,500,000		7,500,000	7,500,000
Recognition of milestone		7,300,000
Sanofi S.A. | Collaboration and license agreement | Milestone Three
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Milestone revenue receivable		13,500,000			$ 13,500,000
Recognition of milestone		100,000
Pfizer SB-525
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues under agreement		55,000,000
Collaborative arrangement transaction price		134,000,000
Revenues		0
Pfizer SB-525 | Collaborative arrangement | Change in collaboration agreement scope
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenues		1,600,000	8,900,000	5,700,000
Net loss		$ 1,600,000	$ 8,900,000	$ 5,700,000
Earnings per share, basic (in dollars per share) | $ / shares		$ 0.01	$ 0.06	$ 0.05